1. Field of the Invention
The invention relates to film deposition systems and, more particularly, to a process and apparatus for the continuous or intermittent removal of tractable and intractable Langmuir-Blodgett films on a continuous coating trough.
Langmuir-Blodgett films consist of monomolecular layers of amphiphilic molecules deposited on a substrate of choice. The continuous or intermittent removal of unused film is necessary to maintain a continuous system of thin film at a constant surface, or Wilhelmy, pressure.
Monomolecular layers of organic compounds are important in a number of applications, most recently in the electronics industry where they can be used to form precisely determined dielectric barriers between two otherwise adjacent metal surfaces. In addition to general insulating applications, a more recent discovery, accomplished by the invention described herein, is the deposition of a protective coating over the metallic oxide surface of computer diskettes. The protection afforded by the Langmuir-Blodgett films, which must be laid down with absolute precision, permits the read-write heads of a computer diskette drive to ride much closer to the surface of the diskette, allowing for higher resolutive and more accurate reading and writing, of data to and from the diskette.
2. Description of the Prior Art
The traditional, and originally novel, approach for producing monomolecular layers of amphiphilic substances on a substrate is the Langmuir-Blodgett method, wherein a thin film of the substance to be deposited is first dissolved in a volatile solvent and spread over the surface of water within a rectangular basin. The layer of amphiphilic substance which remains after the solvent evaporates is compressed from one end of the water basin to create the desired concentration of substance, while at the same time a substrate is raised through the two phase system at the other end. Ideally, the Langmuir-Blodgett process deposits a monomolecular layer of amphiphilic substance on the substrate.
The resulting surface is either entirely hydrophilic or hydrophobic depending on the polarity of the substrate to be coated. If the substrate is hydrophilic, the hydrophilic heads of the amphiphilic substance will adhere to the surface of the substrate, leaving the hydrophobic tails on the surface, and the resulting surface is thus hydrophobic. The opposite result is obtained if a hydrophobic substrate were dipped.
More recently, more sophisticated methods of laying down monomolecular films have been developed, each of which attempt to create a system which is more continuous. A continuous system would not require stopping the process in order to clean and restore the system as frequently, as is required in traditional Langmuir-Blodgett methods. Some of the inventions allow for the deposition of heterogeneous layers of thin film through the use of multiple compartments or chambers within the trough.
However, none of the prior art devices achieves a truly continuous system in which the unused film is either continuously or intermittantly removed from the dipping chamber - except by completely shutting down the system and draining out the support media - which shutting down prevents the system from being truly continuous. Thus, the inventions of the prior art all suffer from a general inability to maintain a continuous film of uniform pressure, consistency and viscosity. Furthermore, none of the prior art discusses how to control the "dwell-time" or "residency time"--the time during which the film remains on the surface of the water--which control is necessary in order to achieve the results of this invention.
For example, Barraud, U.S. Pat. Nos. 4,093,757 and 4,224,896, describe a method and device, respectively, which are slight modifications of the original Langmuir-Blodgett method. The water trough is divided into at least two compartments, each divided from the next adjacent compartment by either a rotating cylinder or torus, which act both as a barrier and a means to transfer the amphiphilic film to the next adjacent compartment. The last cylinder (or torus) in the series also compresses the film within the final compartment in which the substrate is to dipped. No means for film removal other than by dipping a substrate are claimed or specified.
Barraud, U.S. Pat. No. 4,511,604 sets forth a process and apparatus for producing alternate monomolecular layers which utilize two separate, but simultaneously operating tanks, containing separate monomolecular films of either the same, or varying, molecular structure. The Device sequentially dips the substrate into the two separate tanks, thus applying alternate monomolecular layers to the substrate. No means for film removal other than by dipping a substrate are claimed or specified.
Barraud, U.S. Pat. No. 4,632,800 specifies a process for producing a film having at least one monomolecular layer of nonamphiphilic molecules, which is achieved by combining or forming a complex between non-amphiphilic and amphiphilic molecules, a monomolecular layer of which is then placed on a desired substrate. No new device for laying down the film is claimed or specified.
Miyazaki U.S. Pat No. 4,674,436 teaches a device for applying at least two different films of varying chemical makeup without the necessity of cleaning the apparatus between batches. This is achieved by dividing the supporting water surface into four different chambers, separated from one another by floating arms. However, no method for removing unused film is either claimed or taught.
Saito U.S. Pat. No. 4,716,851 describes an apparatus utilizing the traditional Langmuir-Blodgett method of forming thin films, but which has the ability to lay down successive monomolecular layers of film on an already curved surface in order to alter the shape and form a new curved surface containing carefully placed layers of thin film. The patent does not contain any information or claim relating to the removal of unused film.
Albrecht, U.S. Pat. Nos. 4,722,856 and 4,783,348, set forth a method and apparatus which contains a series of compartments similar to Barraud, U.S. Pat. Nos. 4,093,757 and 4,224,896, but which uses a flow of water to transfer the film between compartments and also to compress the film. Albrecht claims an advantage over Barraud because it uses water instead of a moving cylinder to create the desired surface pressure, obviating the need to be retooled when using a different type of film. Although U.S. Pat. No. 4,722,856 claims to be a "continuous" system, it is not continuous in the same sense as the present invention because, like the other prior art, Albrecht neither describes nor claims a means for the continuous removal of unused film in order to maintain a truly continuous flow.
Ono, U.S. Pat. No. 4,779,562, teaches a simple variation of the traditional Langmuir-Blodgett method in which the thin film to be deposited is raised and lowered by the supporting liquid rather than the substrate. Regardless of whatever advantage this process might achieve over other methods, One does not provide any means for the continuous removal of unused film.
Wakayama, U.S. Pat. No. 4,848,270, describes a system for the heterocyclic buildup of a variety of different thin films on a single substrate which is comprised of a multi-compartment system, each containing its own film. It is substantially similar in principal, although different in design, to U.S. Pat. No. 4,674,436, which Wakayama failed to cite.
In all of the devices set forth above, there is a tendency for film remaining in the system after the dipping of one or more substrates to undergo hydration, become contaminated, or otherwise be of a different consistency or viscosity from the film being "continuously" fed into the dipping chamber.
The present invention requires no stoppage of the mechanism in order to clean up and/or restore the system to the desired specifications. The systems of the present invention are "continuous", and are capable of automation. The present invention is therefore uniquely suited for large scaled industrial applications requiring processes capable of producing thin films of precise thicknesses.